One present commercially implemented EAS system of the assignee hereof has a transmitter which radiates a pulsed magnetic field into a surveillance area wherein it is desired to note the presence of articles bearing EAS tags, also referred to in the EAS industry as labels or markers. When a tagged article is present in the surveillance area, its tag is excited by the radiated magnetic field and, based on its composition, is caused to generate a detectable response signal. A receiver, which is enabled between successively spaced transmitter field radiations, detects the response signal of the tag and initiates an alarm or other activity to indicate the presence of the tag in the surveillance area.
EAS systems are commonly installed in environments with high levels of electrical interference, such as retail store checkout areas. Interference sources commonly found in these areas include such items as electronic cash registers, laser product code scanners, electronic scales, coin changers, printers, credit card verifiers, point of sale (POS) terminals, neon signs, fluorescent and halogen lights, conveyor belt motors and motor speed controllers, and others.
The electrical noise environment presented to an EAS system in a retail checkout area is rarely constant. Various electronic devices in the area, such as those listed above, are turned on and off throughout the day, causing an ever-changing pattern of interference, both in the time and frequency domains.
Conventional techniques of filtering, such as band limiting and frequency notching, require extra hardware and often do not eliminate the interfering signals. They rely on improving the desired signal-to-noise ratio (SNR) by attenuating undesired out-of-band signals, while amplifying signals of interest, namely, tag signals.
Time domain approaches, such as receiver blanking and time window masking (discussed below) are effective, but have the drawback requiring extra hardware. Further, when the receiver is blanked or masked, it is incapable of responding to valid tag signals.
Another known practice for addressing electrically noisy EAS environments is the use of a phase canceling receiver antenna scheme. The most common scheme makes use of a FIG. 8 antenna configuration, wherein two substantially identical antennas are connected either in series or parallel, such that signal sources at a distance generate magnetic flux that cuts both coils equally, inducing equal and opposite currents in the coils. When the currents from the coils are summed, they cancel and the net amplitude from the distant source is reduced. This method of noise cancellation is very effective for many types of interference, but has a significant disadvantage in that a tag placed on or near the plane of symmetry between the FIG. 8 receiver pair also has its signal canceled, i.e., the tag is said to be in a receiver null zone. At times, environmental interference is so severe that the presence of null zones represents an acceptable compromise.
Frequency band limiting, done by filtering, is also an effective means of reducing noise interference. System receiver input filtering selectively passes certain frequencies which include the expected tag frequency characteristics and suppresses or blocks frequencies outside of the passband. However, interfering signals have frequencies near the expected tag frequency and are within the passband and are processed in the receiver.
Limiters and noise blankers also have seen use in addressing environmental noise, addressing high level and particularly short duration impulse noise (noise spikes). However, under certain conditions, tag signals can erroneously activate these circuits, causing them to block the desired tag signals.
The commercial EAS system of the assignee hereof above referred to generates a pulsed magnetic field in the form of short bursts of magnetic flux at a frequency to which the system tags are sensitive. The system tags are magnetically resonant at the particular system frequency and because of their significant Q, they will continue to respond or "ring" after the transmitter field is removed. This ringing response is unique and is detected by the system receiver. To protect the sensitive receiver circuitry from being overwhelmed by the high level transmitter field, the receiver circuitry is gated off until shortly after the end of the transmitter burst. For this reason and to prevent interaction between systems, this transmitter burst and receiver window must occur at precise points in time, commonly referenced to the local power line's zero crossing.
Because of the possibility of neighboring systems being powered by different phases from the local power lines, three distinct transmit/receive windows are provided for in the systems' timing scheme, each 120 degrees apart in phase. This strict timing sequence must be adhered to in order to prevent undesired system interaction. This critical timing system has the advantage that noise spikes and impulsive noise occurring at times when the receiver is gated off do not interfere with the system. The processor in the system routinely monitors the background noise for all receiver antennas in all three possible receiver phases. A composite noise average is computed and receiver gain is adjusted up or down to optimize system sensitivity with a varying noise environment. As the background noise average increases, the receiver gain is reduced to allow a defined signal-to-noise ratio to be met without danger of linear stages clipping.
Some repetitive impulsive noise sources can produce interfering signals during receiver windows however, so the system provides for time window masking, which prevents these high noise windows from being included in the average and reducing system sensitivity. Setting this time window masking is a manual step performed at the time of system installation or during servicing of the system.
Once a receiver window is masked, noise during that period no longer affects the average, but the window can no longer be used to process tag signals. If the impulse noise source changes its phase relationship to the power line's zero crossing, such as if the source is another piece of electronic equipment which is relocated or replaced with another unit, its interfering signal now can occur during a non-masked receiver window, reducing system sensitivity, and the masked receiver window is not freed up for system use.